Steam-based bitumen recovery processes are most likely to burn natural gas as the fuel of choice to produce high-pressure steam for bitumen recovery. With the cost of producing steam the single greatest operating expense of bitumen recovery, the overall cost is greatly affected by the price of the fuel used in producing steam. Thus, the use of natural gas as a fuel for producing steam reduces operating costs when the price of natural gas is low but these costs will increase proportionally as the price of natural gas increases. As a result, interest in alternate fuels is particularly kindled when the price of natural gas increases. Previous studies have indicated that bitumen bottoms (resid, asphaltenes, etc.) are competitive as fuel with natural gas when the price of natural gas is higher than a certain level.
Investigations into the concept of using the whole or a separated component of the produced bitumen as an alternate fuel have been made for several years. Past investigators have generally focussed efforts on whole bitumen emulsification as opposed to efforts directed to separating or splitting the bitumen into distinct heavy and light fractions. Splitting the bitumen into heavy and light fractions can produce a higher value, lighter overhead fraction that can be marketed separately as a medium sour crude or be blended into the overall diluted bitumen pool and, a lower value residuum fraction that can be utilized as a fuel and preferably as an on-site fuel for steam generation.
Processes for splitting bitumen into two or more fractions are currently commercially available. Such processes include conventional fractionation with atmospheric and vacuum towers, or solvent de-asphalting to produce the desired resid fraction.
In conventional fractionation, the individual fractions are often cut sharper than might be necessary for an alternate fuel application such as an emulsion fuel for steam generation. Fractionation is also very cost intensive due to the equipment investments required. In fractionation, a crude oil is introduced into distillation columns, usually in a two-tower configuration. The first distillation tower operates at near atmospheric pressure (slightly positive) while the second tower operates under vacuum—the net pressure being a function of the crude oil type and the desired properties of the residuum fraction at the bottom of the column. The distillation columns are designed to maximize recovery of the valuable products from the crude oil, such as, gasoline, jet fuel, diesel, etc., and to recover the products as close to specification cut points as possible. Certain product specifications will require multiple trays in the towers as well as a condensed reflux stream from the top of the tower to strip the individual side streams of the heavier boiling components to meet boiling point specifications of the various products such as gasoline and jet fuels. The trays (or packings) represent a number of theoretical stages (vapor-liquid equilibrium) that are required to meet the cut point specifications of the individual streams.
In solvent desasphalting, bitumen is separated into heavy and lighter components with the addition of a light paraffinic solvent, typically a pure component such as propane or pentane, at a high solvent to bitumen ratio to separate the heavier asphaltic fraction into a distinct phase. Effective storing, recovering and recycling of the solvent is very energy and cost intensive.
A flash operation is distinct from a fractionation/distillation operation in that it only provides one theoretical stage per vessel. Within a flash operation vessel, no trays, reflux or side streams are present but rather only a top and a bottom product stream. The lack of trays, and side and reflux streams makes for poorer boiling point cut properties for the product streams. As a result, there may be significant overlap between the back-end of the lighter stream with the front-end of the heavier stream which for almost all petroleum refinery operations will not be commercially acceptable. However, for certain product requirements, flash separation is satisfactory if the individual product streams do not have to meet precise product specifications.
However, while it has been known that flash operations could potentially be used to separate bitumen, such technology has not been implemented in view of significant operational problems when separating bitumen. In particular, in a two-stage (atmospheric and vacuum) flash operation, there is a significant risk of the higher boiling point compounds migrating to the lighter stream overhead as a result of the lack of trays and/or reflux (particularly in the vacuum stage) which may result in plugging overhead lines. Thus, as a result of both the cut point specification and the risk of operational problems, the use of a highly viscous and asphaltic crude, such as Cold Lake or Athabasca bitumen per se has not been considered for flash operations.
Further, and with respect to the use of solvents for separation, a source of solvents not previously considered for bitumen separation is gas plant condensates, also referred to as gas plant diluent since the material is used to dilute the bitumen for transportation. Gas plant condensates (diluents) are used to dilute bitumen for transportation and generally include mixtures of paraffmic C4-C10 hydrocarbons as by-products of natural gas processing plants. During natural gas processing, various contaminants are removed through condensation to produce a significant volume of these by-product hydrocarbons. Gas plants are often located in relative proximity to bitumen recovery operations and, thus, can provide a ready source of solvents for use in a bitumen separation process.
After separation, the handling and burning of the heavier resid fractions is also difficult. Generally, the resid viscosity is too high to be pumped in its neat form due to its high density and viscosity requiring that the fraction be heated to 200° C. or higher.
Burning the resid fraction is also difficult in burner systems currently available as atomization of the fuel and the temperature at which it becomes amenable to atomization cracks the resid, leading to coke lay down and fouling of the fuel delivery system of the burner. Burning requires that the fraction be heated to over 325° C. in order to lower the viscosity to about 25 cP needed for atomization within a combustion chamber. Experimental work indicates that the resid starts to smoke at a temperature of about 280° C. and crack at about 300° C. Furthermore, and from a practical perspective, in order to obtain a bulk resid temperature of about 325° C. for atomization, the wall temperature of the storage vessel and/or the distribution piping has to be considerably higher than 325° C. which will result in wall coking of the storage vessel and/or distribution piping.
As a result of both the handling and burning problems, it has been known that one method to reduce the viscosity of various bitumens and/or their fractions is to create emulsions of bitumen in water. Emulsions break down the bitumen into small droplets which are dispersed in a continuous phase of water, thereby lowering the apparent viscosity for ease of pumping and transportation. Asphalt-in-water emulsions have been commercially produced for decades, with variations in composition and formulation designed to match specific end uses. However, in the past, the creation of stable emulsions of the heaviest bitumen fractions and particularly those fractions having a high softening point or density have required high temperatures and pressures to create and maintain a stable emulsion and have not been practical. Thus, there continues to be a need for methods which enable the heaviest fractions of bitumen to be formed into stable emulsions suitable for pumping, handling and burning.
In summary, while separation of bitumen into heavy and light fractions is known for various products, there continues to be a need for effective and efficient bitumen separation, handling and separation techniques for producing heavy or resid fractions of bitumen for use as an emulsion fuel. More specifically, there has been a need for an efficient fractionation process as well as processes to render the resid pumpable in order that the resid may be used as fuel for use as a component in an integrated process of steam-based bitumen recovery where a portion of the recovered bitumen is used as fuel to create steam for recovery of the bitumen.
A review of the prior art reveals that such processes have not been proposed. For example, the paper “Bitumen Utilization via Partial Upgrading and Emulsification” (Sankey, B. M., Ghosh, M., and Chakrabarty, T., 6th UNITAR International Conference on Heavy Crude and Tar Sands, Vol. 2 p. 269-276, Feb. 12-17, 1995) describes the concept of splitting bitumen and emulsifying the resid. This paper does not, however, disclose a two-step flash separation or the use of gas plant diluent to separate asphaltene.
Further patents have issued for the emulsification of various heavy oils and bitumen, but not for the type of recalcitrant resid used in this invention. For example, U.S. Pat. No. 4,666,457 describes emulsifying heavy oils in water, primarily for use of bio-emulsifiers and U.S. Pat. No. 6,113,659 discloses emulsifying heavy oil which is softer than the resid fraction of this invention.